Every toyhauler we've purchased has come with the generator already installed by the dealer. So that's something you could negotiate in to a purchase price. When we switched to a traditional 5th wheel, it only came with the prep and we had a generator installed by a third party referred to us by Cummins as it was about $1000 cheaper than the dealer doing it. As you mentioned, it is not cheap. The plus of the toyhauler built in generators is it can use the fueling station with gas. We now have a propane one so it will be a bit more of a pain to "refuel".
The #1 reason we have the generator is for air conditioning. We do a fair amount of warm weather boondock camping in higher elevations (aka the sun is pretty intense) so it was a must for us as we leave our dogs in the trailer when we leave the site. If we didn't need the AC (ie camped in cooler temps or in areas with lots of shade) we could totally get by with a basic solar system and maybe a cheaper gen as a backup.